1. Field of the Art
Manipulating, drying, conditioning or shaping continuous or cut sheet surfaces and surfaces of irregularly shaped objects. Examples include drying, curing, treating, plating, coating, etching, polishing and chemical polishing operations. Though specifically applicable to inkjet printing, the techniques are applicable in almost any surface drying, conditioning, manipulating and shaping situation of various materials that benefits from any of: high efficiency, uniformity, low cost, non-contact manipulation and, or conditioning, and controlled and uniform thicknesses. The techniques are especially useful in increasing the rates of diffusion limited processes at surfaces.
2. Description of the Related Art
Processes today are often limited by the speed at which they condition a surface or medium, which processes often require that the conditioned side not be touched.
In one example application, plating baths currently require close, uniform electrode spacing, high diffusion rates of reactants to the surfaces, and good temperature control. Typical existing plating baths incorporate large tanks which must maintain adequate stirring to maintain uniformity of reactants, but chemicals are wasted because very little of the reactants are actually adjacent the substrates, and energy is wasted due to large electrode spacing and bath heating requirements. In another application, in low cost inkjet printers, the printer mechanism waits for ink to dry or cure sufficiently on a previously printed sheet before adding the next sheet to the output stack to avoid smearing of the ink on the previously printed sheet. Typically low cost printers just wait until the ink dries on the previously printed sheet, even though the print mechanism is capable of printing much faster. Printers which print both sides of the page typically wait even longer for the first printed side to dry before the paper is put through a reverser so the second side can be printed since the reverser mechanism tends to smear the ink on the first side if the ink is not dry. Thus printers that print on both sides of the paper print far slower than printers that only print on one side of the paper. Drying and solidification of inks is limited by the slow diffusion rates of solvents away from the media, and also by slow rate of diffusion of the required heat of vaporization from room temperature air to the evaporatively cooled media.
Higher cost/price printers add various heating mechanisms, including heated platens (sometimes with vacuum hold downs to increase heat transfer rates) and radiant heating means with very little of the radiant heat actually being absorbed by the ink. To date, these methods have been costly, bulky, and inefficient, prohibiting their use in small, low cost applications, such as small office and home printing.
Many prior art drying/fixing/conditioning methods include, individually or in combination, one or more of the following:                1. Drying with a jet of air while the media is suspended between two rollers;        2. Heating the media by means of contact with a hot platen;        3. Heating the media through some form of radiation (typically microwave or infrared); or        4. Introducing, via spray or vapor, fixative materials or catalysts that immobilize or otherwise treat the surface of the media and materials adsorbed thereto.        
However, all prior art methods have one or more of the following limitations:                1. The apparatus used to manipulate the media cannot support the media without contacting at least one side of the media;        2. The system requires the use of continuous media;        3. In drying, considerable heat is wasted because the media does not absorb a substantial portion of the heat used;        4. Friction is introduced into the media path which hinders, or renders unreliable, the media transport process, at high speeds;        5. The dryer/conditioner apparatus is inherently complicated and therefore expensive and unreliable;        6. The drying fluid flow disturbs other processes. For example, in inkjet printing, ink droplets are deflected from their intended target on the media;        7. The apparatus encloses the media, and is therefore bulky;        8. Drying/conditioning is a compromise between what is desired, and what is possible, requiring changes in other parts of a system to accommodate such deficiencies. For example, in inkjet printers, generally inks are carefully designed to dry as fast as possible because the drying apparatus is marginal, and therefore the ink composition often includes surfactants which imply a trade-off between the ink composition required to produce fast drying and that required to produce sharp edge acuity and vibrant color of the image;        9. Prior art systems still have to wait for drying/conditioning despite the improvements that have been made;        10. It is not possible, or it is expensive, to recycle used materials or heat; or        11. The apparatus shape cannot be configured to accomplish, simultaneously, other functions in addition to drying/conditioning, such as flattening the media, or transporting and reorienting, or warehousing the media.        
Low cost printers are capable of depositing ink completely covering a page at about a 30 page per minute rate. However, they never actually print at that rate because the ink takes at least 10 seconds to dry adequately before a successive page can be stacked upon the previously printed page. Inkjet printer manufacturers have been unable to make inks that do all of the following:                1. Dry more rapidly than about 10 seconds on the printed page without the use of expensive, power intensive, and bulky driers, or volatile solvents;        2. Have sharp edge acuity when printed;        3. Have dark blacks and vibrant colors; and yet        4. Do not dry out, and clog nozzles of the printhead when the printer is not in use.        
Typical inks are made with a water carrier, which is environmentally safe, and whose chemistry with respect to pigments and dyes is well understood. The inks further contain surfactants to help the ink penetrate into the paper, humectants to keep the ink moist in the printhead, dyes or pigments for color, and pigments for black. There are often deliberate chemical interactions between the inks to keep one ink from bleeding into another on the paper. A worst case blacked out page at 600 dots per inch of 5 picoliter black dots has about 0.16 cc of ink on the page. The water in the ink sinks into the paper in about 5 seconds, and begins to swell the paper fibers about 1 percent, causing the paper to bow toward the side with ink on it, causing what is known as wet cockle. If the ink is deposited in a swath of a width W, surrounded by dry paper, the paper buckles in a bubble shape about diameter W, and height of about 0.1 W, after, typically, 20 seconds. As the water further penetrates the page, the backside of the paper also begins to swell, tending to flatten, then reverse the direction of the bubbles as the front side dries somewhat, and the back side is being penetrated by water. Eventually the paper is uniformly swelled within the wet swath, and buckles alternating positively and negatively along the swath length, i.e., the width of the paper. As the water in the paper becomes uniformly distributed, and then evaporates, in a minute or more, the fibers tend to return to their former length, but the paper fiber bonds have yielded, and the paper does not return to a completely flat shape leaving residual dry cockle.
Wet cockle can cause a head crash, where the paper buckles enough to hit the scanning printhead, often located about 60 mils above the paper surface. Limiting the size of the swath, and the amount of ink put on the page, can minimize the height of wet cockle, but smaller swaths result in lower print speeds, and less ink results in less dark blacks or less vibrant colors.
Dry cockle is evident in unsightly wrinkled pages and is to be avoided.
Generally the black ink pigment is intended to stay on top of the paper to produce the darkest blacks, with optical densities of 1.3 to 1.4, comparable to offset printed inks. Black pigment inks cannot contain surfactants to the extent that the pigment wicks across fibers, since that would result in jagged edges on letters which is highly undesirable. Thus the black ink pigment is susceptible to smearing, since it is on the paper surface and mechanically in contact with the next sheet of paper which will be stacked on top of it. Though pigments tend to coalesce and solidify when the water carrier is drawn into the paper (after at least 5 seconds), the pigment is often comprised of block copolymers similar to latex paint, thus pigments do not become permanent for days.
Color inks are typically dyes in water solutions with surfactants which help the water penetrate the paper more rapidly. Color inks take less water to cover a region than black inks because the surfactant spread the ink, and because the color inks do not have to be as dense as black is for text. Since the human eye is not as sensitive to color, jagged edges on color droplets are less objectionable. However, color inks would be more vibrant if they were on the surface. One approach would be to use color pigments but pigments typically are ground up solids with particle sizes over 0.1 micron and therefore scatter all colors to some extent making them somewhat duller than dyes that are confined to the surface.
Thus both black pigments and color dyes benefit by being dried rapidly before they can penetrate the surface of the paper. And, problems of paper cockle would also be relieved if paper could be dried substantially in less than 5 seconds (less than 2 seconds for a 30 page per minute printer).
This problem of drying at greater than 30 pages per minute has been continually studied, and to date has not been effectively solved in a way that is suitable for small (less than 1 cubic foot), low cost (less than $100), printers or even printers that are 5 times as large, and 5 times as costly, and ⅕ the speed.
Some ineffective solutions in the prior art include:
U.S. Pat. No. 6,305,796 by Szlucha et al., which discloses an enclosure wherein paper is heated with radiant heat from infra-red bulbs within a reflective enclosure. The enclosure itself is a substantial part of a cubic foot in dimension, the heat required is substantially more than 180 watts due to bulb and absorption inefficiencies, and the paper drying time is longer than the 2 seconds required at a printing rate of 30 pages per minute.
U.S. Pat. No. 6,463,674 by Meyers et al, which discloses an air impingement drying system that also fully encloses the paper, and, because of the large air boundary layer inherent in the geometry, Meyers system is inadequate to meet conditions stated above in the discussion of the Szlucha patent and only slowly dries the paper.
U.S. Pat. No. 6,382,850 by Freund et al., which discloses a large, complex system of heaters and air knives disposed along a 10 inch linear vacuum belt with the paper being held by the back side. In the Freund system the paper is moved at 5 cm per second, therefore drying at only a 12 pages per minute rate.
U.S. Pat. No. 5,510,822 by Vincent et al., which discloses a heated platen which physically contacts the backside of the paper, and the paper is held in close contact to the platen by a vacuum which is only released to allow the paper to move. This has a high enough heat transfer rate, but would require, at a 30 pages per minute printing rate, that the vacuum hold down pressure be released and restored at least 4 times per second (for a 1 inch swath print mechanism), and it has no provision for adequate air movement to dry the ink.
None of the above are suited to simultaneous double sided printing since they all hold one side of the paper in the drying process.